Fluid Catalytic Cracking (FCC) is the most important conversion process used in petroleum refineries. It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefinic gases, and other products. Cracking of petroleum hydrocarbons was originally done by thermal cracking, which has been almost completely replaced by catalytic cracking because of its adaptability towards improved and selective cracking to produce high octane gasoline and valuable lighter olefinic gaseous products. This process also provides flexibility to be tuned to different modes of operational window in order to maximize the product of interest.
The conversion section of FCC unit consists of riser, reactor, stripper, regenerator and their associated hardware internals. The feed is injected into the up-flowing catalyst at the bottom of the riser. Steam is introduced along with the feed for proper atomization. The feed molecules are cracked inside the riser when they are contacted with hot regenerated catalyst producing product vapors and coke. The catalyst activity is reduced due to deposition of coke on the catalyst. The cracked products along with the catalyst move up through the riser and the primary disengagement of the catalyst from the hydrocarbon vapor is achieved through riser termination device. The hydrocarbon vapor after separation from catalyst is fed to a fractionation section for separation into various cuts. The separated spent catalyst is steam stripped in stripper to recover trapped hydrocarbons inside the catalyst pores. The stripped spent catalyst flows to regenerator wherein the coke deposited on the spent catalyst is burnt off in presence of air and/or oxygen containing gases to restore the catalyst activity. The hot regenerated catalyst is subsequently recycled back to the riser bottom to complete the cycle.
The demand for light olefins like ethylene and propylene as building blocks for the production of petrochemicals will continue to grow. Propylene demand growth rate outpaces ethylene due to high demand of poly-propylene and other propylene derivatives. The conventional steam cracking units, which are more energy intensive, cannot meet the incremental demand of propylene as its propylene to ethylene ratio is low. Furthermore, much of the new steam cracking capacity is based on ethane feed, which produces little propylene. Therefore, although steam cracking continues to supply most of the world's propylene, there is an increasing need for production of propylene from other sources.
U.S. Pat. No. 5,348,642 discloses a catalytic cracking process and apparatus wherein a part of hot regenerated catalyst is passed directly from the regenerator to the stripping zone via a conduit to increase the stripping zone temperature resulting in improved recovery of hydrocarbons from the spent catalyst.
The conversion of the feed-stocks processed is optimized as per the product slate requirement by designing the reaction of severity, i.e. reactor outlet temperature (ROT) and catalyst to oil ratio (C/O). In a FCC unit, ROT is the measured variable which is achieved by controlling catalyst circulation rate (CCR) to the riser bottom from the regenerator vessel. At constant feed rate, increase in ROT leads to a higher CCR and thus a higher C/O inside the riser. There are prior art inventions where the C/O has been increased without changing ROT.
U.S. Pat. No. 5,597,537 discloses a FCC apparatus which mixes a part of spent catalyst with regenerated catalyst in a separate chamber to obtain a blended catalyst stream before contacting with feed. Mixing of the spent catalyst (normally at lower temperature) with regenerated catalyst (normally at higher temperature) in the mixing chamber results in a lower equilibrium temperature at the riser bottom leading to increase in catalyst circulation rate (CCR) at a given reactor outlet temperature (ROT). However, this prior art doesn't teach cracking of C4 hydrocarbons to C3 olefins.
U.S. Pat. No. 8,163,247 discloses process for contacting feed with mixed catalyst in a secondary reactor that is incorporated into an FCC reactor. The mixed catalyst used in the secondary reactor is regenerated catalyst from a regenerator that regenerates spent catalyst from an FCC reactor that is mixed with spent catalyst from either the FCC reactor or the secondary reactor. The mixing of spent and regenerated catalyst reduces the catalyst temperature and tempers catalyst activity to inhibit both thermal and catalytic cracking reactions.
US20040060846A1 discloses a deep catalytic cracking process to produce increased yields of C3 and C4 olefins at the expense of C2 olefins. In this invention, the riser reactor is configured to have two different radii in order to produce improved selectivity to C3 and C4 olefins as products. In the second broader riser section, Weight hourly space velocity (WHSV) is significantly lowered, so that gasoline range molecules produced in the first narrower section is cracked to produce high yields of the light olefins. However, the prior art does not teach any method that is directed specifically to conversion of C4 hydrocarbons to C3 olefins.
U.S. Pat. No. 7,374,660B2 discloses a process for selectively producing C3 olefins from a cracked naphtha stream. A stream rich in C4 olefins is recycled to a dilute phase reaction zone in the stripping zone separate from the dense phase of the stripping zone to improve the propylene selectivity. However, this prior art doesn't provide any means for achieving optimum temperature and catalyst activity which facilitates maximum propylene yield.
WO2013/054173 covers the process for the production of propylene from cracking of C4 fraction in FCC. For production of propylene from cracking of C4 fraction in FCC, weight hourly space velocity (WHSV), temperature and catalyst activity play the major role. These parameters vary depending on the position where and how the C4 fraction are cracked within FCC. In this invention, an optimum condition of weight hourly space velocity (WHSV), temperature as well as catalyst activity is achieved for enhanced production of propylene from cracking of C4 fraction. This is achieved by cracking C4 hydrocarbons in a reaction zone of optimum weight hourly space velocity (WHSV) (stripper bed) where optimum weight hourly space velocity (WHSV) is achievable. Optimum temperature (higher) and Optimum catalyst activity is achieved in this zone by injecting a part of regenerated catalyst directly into the reaction zone using an additional catalyst transfer line.
WO 2010/067379A2 discloses a process for manufacturing propylene and ethylene in increased yield by cracking an olefinic naphtha stream and main hydrocarbon stock in combination with an olefinic C4 hydrocarbon stream in different zones of one or more risers of an FCC unit. The olefinic C4 hydrocarbon stream is cracked in the acceleration zone of the riser at 600 to 800° C. and by injecting it at the riser bottom in place of lift stream.
U.S. Pat. No. 4,966,680A discloses an integrated catalytic cracking process for upgrading light olefinic crackate gas from a fluidized catalytic cracking unit by integrating with a separate oligomerization reactor. The olefin containing gases from the FCC is processed in the oligomerization reactor to convert the said olefins to gasoline range hydrocarbons and the alkane rich byproduct gases are used as life media in the FCC riser bottom.
In the above prior arts of “Propylene production through C4 cracking”, it is tried to achieve a higher temperature for efficient cracking of these C4 molecules. But it is seen that beyond certain temperature, the propylene production doesn't improve. The temperature where the C4 fraction is injected is even higher than that used for cracking of the main hydrocarbon feedstock.
As evident from different literatures, in the reaction steps during conversion of C4 fraction to C3 olefin, oligomerization of C4 olefins is the primary step followed by catalytic cracking to produce C3 olefin. Since the oligomerization step is exothermic, lower temperature will facilitate the desired oligomerization reaction. However, prior art processes utilize the same reaction zone, which is operated at elevated temperature.
The present invention relates to a new process and apparatus for catalytic cracking of hydrocarbon feedstock to produce higher yields of propylene, where-in conversion of recycled/external C4 hydrocarbon fraction has been improved thus producing an overall augmented yield of C3 olefin.